THE ADDER. 117 
is found to obtain. So, then, it is necessary to look 
still further to explain the problem. 
5, Sex.— Havine briefly considered the possible 
effects of climate and locality, and having found little 
or no explanation in these factors, note next the 
influence of sex and then of age as agents in this 
production of colour variation. In these two factors 
are to be found, I believe, the most important modify- 
ing influences. First as to sex. 
This point is very apt to be overlooked by a casual 
or non-anatomical observer, siuply because unless the 
specimen being examined were a eravid female, very 
big with young, the observer would not be aware what 
the sex of the adder was. A certain amount of special 
education in the adder’s structure is necessary to 
decide the question of sex. Now, it is found that the 
sex plays a very definite part in this colour question. 
I have often had an adder brought or sent to me with 
the remark or message, “It is a beautifully marked 
specimen.” When such is the case it almost invariably 
happens that the adder is a male. That is to say, 
speaking generally, the colours of the males are far 
more brilliant than those of the females. There are 
light-coloured males and light-coloured females, but 
the former are brighter than the latter. Also there 
are dark males and dark females, but the males are 
blacker than the females in their markings and on the 
throat. 
More definitely still, a brilliant yellow background, 
